Power Lams - Fact or Fiction?

[markinengland]

I wonder why people call any of the lams in any form of laminated bow "power" lams?
If the lams are all glued up at the same time the only thing the lams are doing is providing the "meat" out of which the bow is made. The outer surfaces of the back and belly will share the vast majority of the forces, so how can the central lam or any other be the "power" lam?
OK, by using variations of the Perry Reflex principle is it possible to move some of the work in the bow to the central lams, but the work is still primarily in the outer lams, and it would appear that most commercial bowyers do not do this.
Surely the central lam(s) are just seperating the critical working lams?
Some talk about the central lam being made of a stiff wood and somehow magically adding extra cast. How? The central lam is tapered and in the primary working parts of the limb is very thin! Surely it cannot really influence the thicker or equally thick belly and backing?
Power lams do offer the bowyer and advantage. A power lam can be made from a sexy and visually lovely and expensive wood, but uses failry little of this wood. using such tapered "power" lams also means the backing and belly can be made more efficiently out of less wood, leaving less waste. The bowyer has a bow that is cheaper, but by marketing the "power", visual and rarity value side of the rosewood central lam or whatever is able to charge more money for bows that are essentially identical to bows using cheaper central lams.
Power lams? Con or real performance advantage?
Mark in England

[tom sawyer]

Good question.

When I talk about a power lam, I am referring to a lam that is either between the handle and belly slat or between the belly slat and backing.  Either way, it extends past the handle and fades and at least a few inches into the working limb.  I find them to be helpful in providing a more gradual step-down during the tillering of these bows.  It may be that I'm not pretillering/tapering my belly lams enough and for this reason I am starting out with a blank that is weakest right at the handle area.  And with the thin belly slats that are sold commercially, you don't often have a whole heck of a lot of extra wood in this area.  In any case, adding some extra thickness to the inner working limb prevents the possibility of putting performance-robbing set in those inner limbs where a little set translates to a lot of deflex at the tips.  As for the name "power lam", I never really gave it much thought.  But this innermost limb area adjacent to the fades, probably feels a good deal of stress and it does contribute to stored energy as a heavy spring that moves very little.  I guess one could make the case that this would justify the name.

[Badger]

Mark, I usually see the power lam reffered to in glass bows but from what I understand the power lam is short and only extends a bit past the riser stiffening the center section a bit effectively lowering the "working" bow mass. I have never heard of the center lam reffered to as a power lam. On the simple backed bows that I build I simply extend the fade a bit further out for a similar effect, on glass bows and some wood laminated bows the lam thicknes is predetermined so an extra lam must be added as a stiffener. Steve

[Badger]

I posted the same time as Lenny i think!

[Justin Snyder]

I always thought along the same lines as Lennie and Steve.  I almost always add the "power lam" to a board bow with a glued on riser.  It helps stiffen the handle section and reduce the possability of the glued on riser popping off. Because it is sandwiched between the belly and back it cannot pop off.  Justin

[DCM]

The only other thing I would speculate is perhaps it refers to the lam which carries the taper.  It's simpler on a multi lam to use parallels and one set of tapers to set the tiller and weight, once you develop a forumla. 

[1/2primitive]

Lately I've talked with Mike Westvang (he's the owner of Dryad bows), and he basically said that it helped to give more 'insurance' against hinging the bow in the fade section. I would think he would know, since he and his son made about 100 bows just in the past year. 
     Sean

[Justin Snyder]

I could be mistaken but unless you really messed up before you glued it up, I just don't see how a power lam could stop a hinge in the fades.  You remove wood from the fades during tillering and that is when a hinge would occur. If you remove to much wood you will get a hinge regardless of the power lam. If you don't remove enough you wind up with stiff long fades.  Justin

[1/2primitive]

Justin, this is the way I look at it;
Most glue ups are made from thin slats, so the power lam just allows more wood in that place, yes, perhaps making it too stiff, making the bowyer take more wood off in that area to get it to bend the proper amount, instead of it maybe already being too weak. You are right, if too much wood is taken off, it will hinge regardless, but I think it just gives more initial strength.
I don't have extensive knowledge in this field, so I'm just sharing what I speculate.
     Sean

[tom sawyer]

Thats right Sean.  Unless you do a lot of tapering/pretilering prior to glueup, what you'll get with a backed bow after glueup is a blank that is weakest near the handle.  Until you get that mid-limb bending you are prone to put set in your inner limb.  You can minimize this by being careful and not overstressing the inner limb, but at the same time you really need to bend the bow some to see where it needs work.  This puts a lot of stress on the inner limb, sometimes too much.

I do think it is more necessary on the commercially available slats that are fairly thin to begin with.  You often get 1/2" or less with these things.  If you started with 3/4" you wouldn't need this extra bit of wood near the handle.  But adding a thin (1/8") 12"-long piece of wood, allows you to be more parsimonius with the slat wood and there is also the advantage that you don't have to remove that extra 1/4" of wood from the 3/4 of the limb where it is unneccesary.   

This extra lam also serves an aesthetic function.  You can use a contrasting wood and it shows up very nicely.  In addition, you can see just where your thickness taper is going.  You want to work it so the power lam tapers and both ends round off at a similar distance to the handle piece.

[mullet]

   I believe the saying came from glass bows as a Marketing tool. It sounds Fast ,Dynamic and cool.

[Rich Saffold]

Like Eddie said . It allows for one to have shorter  working limbs on a longer bow. I have one which is a favorite, but its just one part of what can be a good laminated bow, not a solution in itself.

Rich-

[medicinewheel]

well, mullet is only almost right; i guess he mistakes powerlam for actionwood. that's a layer of vertically laminated wood or bamboo (actionboo).

but if you have a trilam (or polylam) bow you CERTAINLY can increase performance by forcing energy into the neutral center of the bow, these techniques are heard of as 'perry reflex' or 'duoflex', both described in TTB3.
the way i hear the term 'powerlam' being used it always seemed to refer to this technique.
didn't dan perry post here occasionally?? he should be able to explain better!

than again, actionwood can be used as a center laminate in wooden bows very well, and - as hardwood/bamboo combination - it has been used in japanese bows since a thousand years. i tell you it DOES increase performance! (see chris glauben's BOM/BOY-winning bow; that's the same sort of technique)

frank

[markinengland]

Frank,
Surely this "actionwood" is just a power lam called another name? I can't see that there is anything special about a central lam of vertically laminated wood or bamboo. The only way these will increase performance will be if some form of perry reflex is in use or if they allow a lighter bow by replacing heavy belly stave wood with a lighter core. Oddly, here in the Uk the central lam is often a heavier wood. The justification being that this is stiff and therefore somehow resists bending. I can't see the physics behind that!
How do you think this central action wood increases performance? What is going on in there so do this?
Mark

[Justin Snyder]

I think a trilam or multi lam is entirely different.  It could be used in conjunction with a power lam, but would be and entire limb length thing where the power lam is to stiffen the handle/fades area.
Lennie, I guess I can see where it could be a safety thing in a commercial application. The guy building the blank isn't the guy starting to build the bow so he doesn't already know the next step.  I dont pretiller, but I leave the fades and next 4" about 1/16" thicker than the limb so it dont make a difference to me.  Justin